Most oil field elevators are lifted by bails from a traveling block and lift pipe that passes vertically trough an opening extending along the elevator general centerline. Such elevators usually have doors, open like a book, or have a closed peripheral body containing a slip bowl in which slips operate to grip pipe. Slips are effectively wedges and the force of gripped pipe pulls the slips downward in the funnel shaped slip bowl to assure inward wedging of the slips to securely grip pipe. This invention relates to slip equipped elevators.
Oil field elevators that have to lift slips to release pipe strings suspended by the elevator are usually not capable of releasing pipe strings that are pulling down on the slips. To lift the slips and release the pipe considerable upward force is required to first lift the pipe string and then lift the slips.
The apparently reliable fail-safe system can sometimes fail under certain circumstances. Pipe strings have been dropped into wells because the pipe string, moving downward in the well, encountered brief resistance to movement and the elevator moved downward relative to the pipe. Moving upward relative to the elevator, the pipe weight no longer urged the slips closed. That allowed the slips to move upward and create a condition to drop the pipe. To avoid such accidents, a safety latch was provided to lock the slip control in the slips-down position. From the drilling floor, however, it is not easy to determine if the safety latch is in the safe position and some accidental drops continued to occur.
There is a need to provide a driller level indicator that the slip safety latch is in the safe position. There is a further need to avoid adding to the wires that already descend from various derrick apparatus to the drilling floor. In addition, the safety feature should be workable without time consuming drilling rig modifications. The needs represent objectives in pursuing the development of the present invention.